1. Field of the Invention
The present invention relates to a conductive member such as an electrification roller, a transfer roller or the like used in image forming apparatuses such as copying machines, laser beam printers, facsimiles or the like, a process cartridge using the conductive member and an image forming apparatus using the process cartridge.
2. Related Art Statement
A conductive member such as an electrification roller for applying an electrification process to a photoconductor drum or image carrier, or a transcription or transfer roller applying a transcription process to a toner image on the photoconductor drum has been used for a conventional electro photographic type-image forming apparatus such as an electro photographic type-copying machine, a laser beam printer, and a facsimile.
FIG. 14 illustrates a conventional electro photographic type-image forming apparatus having an electrification roller.
As shown in FIG. 14, the conventional electro photographic type-image forming apparatus 120 is composed of a photoconductor drum 101 on which an electrostatic latent image is formed, an electrification roller 102 contacting with the photoconductor drum 101 for carrying out an electrification process to the photoconductor drum 101, an exposure mechanism 103 for a laser beam or the like, a development roller 104 for transferring toner on the electrostatic latent image of the photoconductor drum 101, a power pack 105 to apply a DC voltage to the electrification roller 2, a transfer roller 106 for transferring the toner image on the photoconductor drum 101 to a recording paper 107, a cleaning device 108 for cleaning the photoconductor drum 101 after the toner image is transferred, a surface electrometer 109 for measuring a surface potential of the photoconductor drum 101.
The image forming apparatus 120 has a process cartridge detachable system. That is to say, the image forming apparatus 120 comprises a process cartridge 110 in which a process instrument including the photoconductor drum 101, the electrification roller 102, the development roller 104 and the cleaning device 108 is removably attached to a main body of the image forming apparatus.
The process cartridge 110 may include at least the photoconductor drum 101 and the electrification roller 102. The process cartridge 110 is mounted on a predetermined place of the image forming apparatus 120, thereby the process cartridge 110 is structured to be in connection with a drive system and an electric system of the image forming apparatus, which are not shown.
Meanwhile, a functional unit necessary for another electro photographic process conventionally is not required for the present invention, it is omitted in FIG. 14.
Next, a basic operation of the conventional image forming apparatus 20 of the electro photographic system is explained.
When a DC voltage is supplied from the power pack 105 to the transfer roller 102 contacting with the photoconductor drum 101, a surface of the photoconductor drum 101 is charged uniformly to a high electric potential. Immediately thereafter, when image light is irradiated to the surface of the photoconductor drum 101 by the exposure mechanism 103, the irradiated portion of the photoconductor drum 101 has a reduced potential. It is known that such an electrification mechanism on the surface of the photoconductor drum 101 by the electrification roller 102 is formed by a discharge according to the Paschen's Law based on a minute gap between the electrification roller 2 and the photoconductor drum 4.
Because image light is a distribution of light based on white/black of image, when the image light is irradiated to the photoconductor drum 101, a potential distribution or electrostatic latent image based on a recorded image is formed on a surface of the photoconductor drum 101 by the irradiation of the image light. In this way, when the portion of the photoconductor drum 101 where the electrostatic latent image is formed passes the development roller 104, toner is attached to the portion of the photoconductor drum depending to a high or low potential to expose the electrostatic latent image and form a visible image on the photoconductor drum. A recording paper 107 is sent in a predetermined timing to the portion of the photoconductor drum where the toner image is formed by a resist roller (not shown), and disposed to overlap the toner image on the photoconductor drum.
After the toner image is transferred on the recording paper 107 by the electrification roller 2, the recording paper 107 is separated from the photoconductor drum 101. The separated recording paper 107 is transported passing through a transportation path and ejected out of the image forming apparatus after it is heated by a fixing unit (not shown). In this way, when the transcription or transfer is completed, the surface of the photoconductor drum 101 is cleaned up by the cleaning device 108 and then a residual potential on the photoconductor drum is eliminated by a quenching lamp (not shown), and the photoconductor drum on which the potential is eliminated is prepared for the next image forming treatment.
For a conventional electrification system using an electrification roller, a contact-electrifying system is often used, in which the electrification roller is contacted with a photoconductor drum (see for reference JP-A-S63-149668 and JP-A-H01-267667).
However, there are some problems in the contact-electrifying system as follows.
(1) If a material used for the electrification roller seeps from the electrification roller, the seeped material is transferred and attached to a charged member, for example, a photoconductor drum and hence traces of the electrification roller remain on a surface of the charged member.
(2) When an alternating voltage is applied to the electrification roller, the electrification roller contacting with the charged member or photoconductor drum oscillates, resulting in an electrification noise.
(3) The toner on the photoconductor drum is attached to the electrification roller, thereby electrification performance is decreased. Especially, the toner is easy to attach to the electrification roller by the seeping of the material as mentioned above.
(4) The material for the electrification roller is easy to be attached onto the photoconductor drum.
(5) If the photoconductor drum is not driven for a long time, the electrification roller is permanently deformed.
In order to resolve the above problems, there has been proposed a proximity-electrifying system in which an electrification roller is disposed close to a photoconductor drum without contacting with the photoconductor drum (see for reference, JP-A-H03-240076 and JP-A-H04-358175). With this proximity-electrifying system, the electrification roller is disposed to face the photoconductor drum with the closest distance (within a range of 50 to 300 μm) and a voltage is applied to the electrification roller to charge the photoconductor drum.
However, because the electrification roller does not contact with the photoconductor drum in the proximity-electrifying system, there are no problems such as the attachment of the material of the electrification roller to the photoconductor drum and the permanent deformation of the electrification roller, which are caused by the contact-electrifying system.
In the electrification roller of the proximity-electrifying system disclosed in JP-A-H03-240076 and JP-A-H04-358175, spacer ring members are provided in opposite sides of the electrification roller to hold a gap between the electrification roller and the photoconductor drum.
However, because an improvement for setting the gap accurately is not made for the electrification roller of the proximity-electrifying system, the gap varies by variation of size accuracy of the electrification roller and the spacer ring members, therefore the electrification potential of the photoconductor drum varies, thereby there is a problem that the toner is attached to a white background of the paper when forming the image and therefore a defective image occurs.
In order to resolve the problem in the proximity-electrifying system, there has been proposed an electrification member or roller including tape-like gap holding mechanisms each having a predetermined thickness (see, for reference, JP-A-2002-139893).
However, in the electrification roller including the tape-like gap holding mechanisms, when the electrification roller is used for a long time, because the tape-like gap holding mechanisms wear and the toner enters and fixes between the electrification roller and the gap holding mechanisms, there is a problem that the gap between the photoconductor drum and the electrification roller cannot be maintained.
In addition, in the electrification roller including the tape-like gap holding mechanisms, because the thickness of each of the gap holding mechanisms varies, there is a problem that a high accurate gap cannot be formed.
FIG. 15 illustrates a preceding electrification member proposed by the inventors in the present application.
The electrification member 210 includes a conductive supporting body 201 made of a metal, an electric resistance adjusting layer 202 formed on the conductive supporting body 201, and space holding members 203, 203 provided on opposite ends of the electric resistance adjusting layer 202, made of a resin and configured to maintain a gap between the electric resistance adjusting layer 202 and a member, for example, photoconductor drum facing the electric resistance adjusting layer 202. The space holding members 203, 203 are made of a thermal plastic resin which has a Durometer's hardness: a range of HDD30 to HDD70 or less and an abrasion mass measured by a tapered abrasion machine: 10 mg/1000 cycles or less (see Japanese Patent Laid-Open 2005-024830).
In the electrification member 210, each space holding member 203 is attached to the conductive supporting body 201 to contact with an end surface of the electric resistance adjusting layer 202.
In the electrification member 210, the space holding members 203 can be firmly fixed to the conductive supporting body 201 by applying an adhesive between the space holding members 203 and the conductive supporting body 201, whereby maintaining the stability of the gap.
However, because there is a great difference between linear coefficients of expansion of the space holding members 203 made of the resin and the conductive supporting body 201 made of the metal, there is a possibility that detachment at a boundary surface between the space holding member 203 and the conductive supporting body 201 occurs if the electrification roller is placed in low temperature or high temperature environment. Therefore, the electrification roller lacks in reliability throughout a long period. In addition, an adhesive strength at the boundary surface between the space holding members 203 and the conductive supporting body 201 is decreased by energization for a long time.
When the space holding members 203 are shifted, because the gap between the electric resistance adjusting layer 202 and the photoconductor drum varies, variation in electrification is easy to occur. In particular, a high accuracy for the gap can be accomplished by working the electric resistance adjusting layer 202 and the space holding members 203 simultaneously, but if the attachment for the space members 203 is insufficient, there is a problem that the space holding members 203 are rotated relative to the conductive supporting body 201 when a finishing process such as a grinding process, a cutting process for the electric resistance adjusting layer 202 and the space holding members 203 is carried out.